I agree with all of the above but add that many students, until they are taught, don't have their priorities in order. Ask a new student, "What is the most important thing when starting your motor?" and they something like "avoid hydraulic lock" or some such thing. IMHO, wrong! The most important thing is clearly to keep your hands and fingers out of the arc of the turning propeller and don't let your plane get away from you and hurt someone else. What is the most important thing when landing your plane? It is the same thing as when taking off -- don't hit anyone with your plane. Not keeping safety as the top priority is, in my opinion, the biggest mistake a student, or, experienced flyer for that matter, can make.